The 9120a printer that sits on top of a 9100 calculator seems to be the electrostatic type: that is, the printhead electrically vaporizes a thin coating on the paper's surface to expose a black layer underneath.

I recall these printers from the 70s, but it's been decades since I've seen one or the special paper they use.

I've never had any luck finding a supply of this other than on ebay and only very rarely. While I no longer have a 9100 and printer, I do have a couple of other calculators (Olivetti Divisumma 18 and a Sharp El-8151) that use electrosensitive paper (that's the proper name for it) and they are equally hard to find paper for.

Electrosensitive paper still seems to be used in the medical industry (makes sense, give that the printouts will never fade), but it all seems to be 2 5/16" width rather than the 2.5" width HP specs. Wonder if that would be a problem?